Systems for the Inverse Problem in Cardiology.
Cardiac arrhythmias are one of the main causes of mortality in the world. Current therapies have their fundamentals on a partial knowledge of the mechanisms of the most usual arrhythmias (atrial and ventricular tachicardias, atrial and ventricular fibrillation, and others), and though these therapies reach high levels of effectiveness, the detailed knowledge of a fast arrhythmia (tachyarrhythmia) is the key for creating new anti-arrhythmic therapies or for improving the actual ones.
Nevertheless, the knowledge of the arrhythmic mechanism in a given patient is limited by the fact that the physical magnitude involved is the electric impulse propagation throughout the cardiac cells. The visualization of electric activity in the internal surface of the heart (endocardium) is troublesome, given that current technology only gives indirect measurements, consisting of electric voltage measured in catheters inside the heart (electrograms). These measurements record the electric field that is induced by the cardiac currents at a given distance of atrial or ventricular walls, and hence, mathematical calculations are required for estimated the numerical values of the cardiac currents in the endocardial surface.
Intracardiac navigation systems allow the spatial reconstruction of one or several cardiac cavities and a representation of myocardial electrical activity changes with time, using the electric signal recordings in diverse points and the detection of the spatial location of the catheter from different spatial location media. Currently, several cardiac navigation systems are used to reconstruct the cardiac electric activity in the myocardium from measurements in catheters. The most relevant are the following:                i. Carto System (Biosense, Cordis-Webster). It is probably the most widespread used. It allows to obtain an image (color-coded) of the relative activation time of the endocardium with respect to a reference signal during a stationary rhythm. Its main limitation is it only can be used in stationary rhythms, hence it can not be used in real time, for analyzing the nature of non-periodic arrhythmias. More, it requires a time for mapping the electric activity in each patient, between one and three hours, which represents a high cost for the health system.        ii. Localisa. This system is similar to the preceding one, and it was commercialized by Medtronic. It is no longer commercialized, and its successor is Navex (in the sense that it uses the same system for spatial detection).        iii. Ensite. It is an advanced system allowing the reconstruction of the myocardial electric activation from the recordings in a catheter array. Theoretically, it allows this reconstruction in an instantaneous form, hence being potentially useful for any kind of arrhythmia (periodical or not).        
Probably, the reason why Ensite has not gained wider acceptance and use in practice, despite its theoretical advantages, is that it gives an estimation of bioelectrical currents with an associated uncertainty. Improvement of this uncertainty would make a system of this family having a widespread acceptation in the clinical practice. Other problems are the catheters dimensions, its complicated manipulation, its price, and the fact that the accurate information is limited to the proximal zone of the electrode.
In the current state of technique, several systems are described including the use of catheters for cardiac mapping. Among them, we can consider the patents U.S. Pat. No. 6,892,091, U.S. Pat. No. 5,297,549, and U.S. Pat. No. 5,311,866.